Commercial and private aircraft typically utilize flexible air ducting systems for the movement and transport of air throughout the occupied and pressurized cabin and cockpit areas. These systems, generally referred to as environmental control systems, utilize airflow ducts to circulate filtered low pressure air which has typically been chemically and thermally conditioned. Because such ducting must pass through a virtual labyrinth of aircraft structural components and aircraft systems, typical ducting systems are multiply-curved and often “snake-like” in design and shape. To provide this multiply-curved structure, the majority of aircraft ducting systems are typically fabricated of a combination of rigid and flexible duct components.
For the most part, currently available ducting systems are more or less effective in transporting conditioned air throughout the occupied cockpit and cabin areas of the aircraft. Unfortunately, the ducting networks within the aircraft environmental control systems also tend to function as carriers and conductors of noise throughout the cabin and cockpit areas. Within a typical aircraft in flight, noise is generated internally within the environmental control system by a variety of sources including circulation fans, valves, connectors, rough interior duct walls, turbulence at points of duct connection and differently sized orifices within the system used to constrict and meter air flow. Externally, additional low and mid frequency noises generated by other aircraft operating systems such as hydraulic pumps engine sounds etc often pass through or are communicated to the walls of the environmental control system ducts and into the duct passages themselves. As a result, a difficulty arises in designing an environmental control system which meets all systemic demands of proper air flow and circulation without also allowing undesirable noise levels to be generated, introduced, or otherwise carried into cabin areas.
Noise within aircraft cabin and cockpit areas can be extremely distracting and annoying. In the extreme, unrestricted noises from environmental control system sources may represent a health hazard to air crew and passengers alike and may greatly add to the fatigue of air travel and aircraft operation.
Unfortunately, noise within an operating aircraft is inevitable. The task therefore for environmental control system designers is to minimize the amount of noise carried by or created within the environmental control system duct work. To reduce noise transmission within environmental control system ducts, practitioners in the art position mufflers or noise intenuating devices within the environmental control system ducting at critical points. The objective of such mufflers is to reduce the noise level carried by low pressure air passing through and being discharged from the environmental control system duct. In many aircraft, as many as one hundred or more muffler devices may be utilized within the environmental control system. Accordingly, such muffler devices represent a significant portion of the cost and weight attributed to the environmental control system of the aircraft. Thus, environmental control systems become a significant cost and weight entity in aircraft design.
As a general statement, the material used on aircraft are all subject to overriding requirements of reduced weight and reduced flammability. Weight reduction relates generally to criteria such as performance, strength and cost efficiency while reduced flammability relates to criteria concerning safety. While both weight reduction and reduced flammability are desirable, they are often in opposition. Materials such as metal are excellent for flammability and strength but are often prohibitive in weight compared to other materials. Thus, to reduce weight, many systems and system components within an aircraft must for all practical purposes be fabricated from non-metal materials. Unfortunately, most non-metal materials tend to be flammable and combustible.
The environmental control system of an aircraft and the components used therein are as a result of weight considerations fabricated largely of non-metal materials. In most environmental control systems, components such as mufflers or the like are fabricated of non-metal flammable material and as a result increase the combustible flammable material within the aircraft. The extent of combustible and flammable material aboard and aircraft is often referred to as its “fuel loading”. Materials which are combustible and/or flammable are described as materials which increase fuel loading. Many materials currently used in aircraft mufflers and similar components such as silicone rubber are flammable and therefore require the addition of fire retardants which can reduce fuel loading but which also increase the duct system weight and reduce the mechanical properties of the fabricated muffler. As a result, there exists a direct relationship between the weight of environmental control system mufflers and their flammability. In essence, this relationship relates to the quanta of potentially flammable material (or fuel) which are provided by the muffler to an aircraft fire.
Because aircraft environmental control systems and the mufflers therein contain a continuous flow of air, an onboard fire within the aircraft may be increased by this air flow. Therefore, fires within or near the components of the environmental control system such as mufflers are particularly hazardous. It is desirable therefore to reduce the weights of components such as mufflers and the like to achieve not only the anticipated efficiency of weight reduction but also to improve the flammability hazard within the environmental control system and the aircraft generally.
In attempting to minimize the fire hazard aboard an aircraft, several design criteria and constraints have been imposed upon the ducting systems of aircraft environmental control apparatus. These design criteria and constraints originate generally from governmental and industrial regulations imposed upon aircraft fabrication. Many of these regulations focus upon the safety of aircraft passengers and personnel in the event of an aircraft fire. These constraints include attention to flammability, toxicity and smoke generation during an aircraft fire. Recognizing the need for safety and protection of crew and passengers in the event of aircraft fires, the federal aviation authority (FAA) has implemented a succession of standards and regulation for materials utilized within aircraft environmental control systems. A new and currently developing flammability test is likely to be implemented and is generally referred to as “new radiant panel test” (NRPT) which presents a high standard relative to flammability.
In attempting to meet the complex and often conflicting requirements of environmental control system muffler design, practitioners in the art have provided a variety of systems and devices. For example, U.S. Pat. No. 7,546,899 issued to Tomerlin et al sets forth a LIGHTWEIGHT POLYMER MUFFLER APPARATUS AND METHOD OF MAKING SAME in which a muffler includes a thin wall polyether ether ketone (PEEK) cover tube, an open cell polymer actuator tube slip fit their into and polymer end fitting securing the tubes together and forming a pneumatic seal there between.
U.S. Pat. No. 6,105,620 issued to Haberl sets forth a FLEXIBLE TUBE DEVICE which is bendable and which has the capability of substantially maintaining its shape as bending forces are released. The device is characterized in that it includes a flexible part including a flexible inner hose or a flexible outer hose which may surround the inner hose.
Published patent application US2010/0044149A1 filed on behalf of Patal et al sets forth an ACCOUSTIC MANAGEMENT OF FLUID FLOW WITHIN A DUCT in which sound-dampening apparatus is provided consisting of a duct through which fluid flows such as an air duct. A flexuous cord is helically wound around the inner or outer surface of the duct at a pitch corresponding to a selected acoustical frequency range associated with the fluid flow through the duct.
In a generally related art, U.S. Pat. No. 5,482,089 issued to Weber et al sets forth a FLEXIBLE CONDUIT FOR THE EXHAUST LINE FOR AN INTERNAL COMBUSTION ENGINE which utilizes a flexible supple tube having several helical corrugations of equal pitch and a flexible supporting coil spring both connected to support flanges is provided.
While the foregoing described prior art devices have to some extent improved the art, there remains none the less a continuing need in the art for ever more effective and cost efficient mufflers and methods of manufacture therefore.